After a brief stint on Coronation Street, this was Johnston’s first major role on the soap, which first aired on Channel 4 in 1982.
The acclaimed actress is the first cast member to be announced as returning for the episode, which will be filmed on Brookside Close.
Conceived by Grange Hill and Hollyoaks creator Sir Philip Redmond, the show drew in audiences of nine million viewers at its peak before finishing on 4 November 2003.
The show, set in Liverpool, was known for its hard-hitting storylines and took on issues like rape, bullying and incest.